Seismic failure of river dikes and current trend of its
rehabilitation in Japan are summarized. Serious deformation of dikes was
mostly brought by soil liquefaction. Stretching type of failure observed
where liquefaction took place in shallow depth of ground was recently
noticed. Treatment against liquefaction is essential to mitigate
earthquake inducing failure of dikes.

INTRODUCTION

Global trends of urbanization to lowland may cause serious damage
to society once a river dike fails due to natural hazards such as
typhoon, earthquakes, tsunami, and storm surge, as was seen in the
Hurricane Katrina case in 2005. As intense rainfall tends to drop in
locally limited area due to global warming adding to the recent trend of
urbanization in Japan, the importance of dike role is increasing
particularly in the urbanized lowland area where population and
infrastructure are concentrating.

Not only the rainfall inducing disaster but earthquake inducing
disaster can not be forgotten because the seismicity around Japan is
recognized to have become an active period (Oike 1995). As the
hinterland was highly urbanized as seen in Figure 1, the Yodo river dike
damage during the Kobe earthquake in 1995 gave a big threat to the
society.

[FIGURE 1 OMITTED]

The river dikes have long history in their construction stage, so
it is general that the mechanical properties of embankment material and
ground conditions are not fully known. This makes it difficult to find
out the failure mechanism and governing factors of seismic damage to
dikes when it happens. However, the accumulation of damaged experiences
is revealing the complex mechanism of seismic failure of river dikes.
This paper summarizes the river dike damage recently seen in Japan
(Sasaki et. al 2004).

TYPICAL DAMAGE TO DIKES DURING PAST EARTHQUAKES

It was known that earthquake inducing failure to river dikes often
caused extensive decrease of its function, and in the worst case, the
height of the embankment became to 1/4 of original height. Failed
sections of dikes were mostly accompanied by longitudinal cracks where
step-like discrepancies were often caused.

Such an extensive damage was frequently seen on liquefied ground.
It was also known that sand boiling was often observed along toes of
dikes resting on old river courses. This experience implies that
liquefaction susceptibility of subsoil could be evaluated from
geomorphologic information such as the old river courses even when
enough boring data are not available.

Table 1 shows recent earthquakes which caused damage to river dikes
in Japan. Those cases can be divided into two groups from the
rehabilitation point of view. No soil improvement technique had been
conducted in the restoration works before 1993. But from the case of
Kushiro-oki earthquake, remedial treatment of foundation ground (Oshiki
and Sasaki, 2001) was accepted as a part of restoration works by the
Ministry of Budgeting. The causative reason and mechanism of seismic
failure of dike became to be revealed from the cases after 1993. Two
typical damage are as follows.

Although subsoil liquefaction had been known as a main cause of
dike damage, but it was not recognized until the case in 1993 that the
large deformation of dike is induced by the liquefaction of bottom part
of dike in a particular situation.

Figures 2 and 3 illustrate the damaged section of the 8-9 m high
dike of the Kushiro River. It was noticed first from this case why the
liquefaction of bottom part of dike takes place. The consolidation of
highly compressible peat layer caused dike settlement, and this
settlement had increased saturated zone in the lower part of the dike
which was liquefied during the event (Sasaki et. al 1994).

[FIGURES 2-3 OMITTED]

Figure 4 shows the damaged section of the Shiribeshi-Toshibetsu
River dike during the Hokkaido-nansei-oki earthquake in 1994. In this
case of damage, it was apparent from the traces of sand boiling nearby
the toe of the dike that the large deformation was caused from the
liquefaction in the subsoil layer. It should be noted that large amount
of depression was observed at its crest as shown in Figure 4. Careful
observation during restoration work at this site revealed apparent two
slip surfaces which ran from the shoulders inside the failed dike as
shown in Figure 5.

[FIGURES 4-5 OMITTED]

It was reported elsewhere (Sasaki et. al 1997) that these slip
planes were brought by the stress change in the dike associated with the
change of stress at its bottom boundary due to the loss of shear
strength of the foundation layer by its liquefaction.

This finding implies that it is necessary to take the deformation
of dike into account adding to the large deformation of liquefied
subsoil layer when to estimate the seismic settlement of dike during
earthquakes. Also it should be noted that stretching type of deformation
usually causes longitudinal cracks and depressions of crest.

REMEDIAL TREATMENT OF DIKES AGAINST SESMIC DAMAGE

As was seen in previous chapter, the soil liquefaction is the key
to whether or not trigger large deformation to dikes during earthquakes.
So it is considered that the prevention of the occurrence of soil
liquefaction is the fundamental measure to mitigate dike damage. Most of
the seriously damaged sections in the past shown in Table 1 were
restored by using remedial treatment method to prevent the liquefaction
(Sasaki et. al 2004) as shown in this Table. The remedial treatments to
improve foundation ground against liquefaction are compiled elsewhere
(Japanese Geotechnical Society 1998).

Comprehensive study revealed that the main cause of the damage at
the Torishima section of the Yodo River dike shown in Figure 1 was
subsoil liquefaction. The deformation and settlement of the dike was
considered to be caused along the process shown in Figure 6 (Sasaki et.
al 2004). Therefore this section was restored with conducting Deep
Mixing Method for improving the liquefiable 10 m thick sand layer
beneath the dike as shown in Figure 7.

[FIGURES 6-7 OMITTED]

The finding gained from the case of the Shiribeshi-Toshibetsu River
dike was utilized for the newly constructed Naka-umi dike. Soil profile
at this site showed that liquefaction would be easily triggered during
an earthquake as shown by FL value in the Figure 8. However it was
hesitated to take soil improvement method to avoid the damage for 3 m
high dike in the rural area. So the geo-grid was placed at its bottom to
prevent stretching type of failure as shown in the Figure 9. Most of the
treated length of the dike section performed well without apparent
deformation during the Tottoriken-seibu earthquake in 2000 (Sasaki et.
al 2004).

[FIGURES 8-9 OMITTED]

EVALUATION OF DIKE DAMAGE DURING EARTHQUAKES

As the role of dikes which protect lowland area from flooding
becomes more important than the past, so it is essential to raise the
resistance of them against causative action of natural hazard. Necessary
room for them to be enlarged is limited in urbanized area, therefore
existing dikes must be added ductile quality at their locations. In
order to achieve this demand effectively, it is necessary to know the
weak sections of dike.

To filtering out the weak sections against earthquakes from the
long spanned whole dikes, sections which protect so-called zero meter
area are firstly to be selected. Then the seismic performance of the
dike in the section is to be examined. As the stability of dike is
mainly governed by the occurrence of liquefaction, the subsoil
conditions of these selected sections are to be examined. If the
liquefaction is anticipated, amount of the dike deformation should be
estimated. Crest settlement is taken as the measure of dike deformation
so that the dike function is to be evaluated. And then remedial measure
to reduce the estimated deformation is to be selected.

There are hidden concepts in the background of the selection of
zero meter area as a primary step of the procedure, those are based on
the past experiences in Japan that the damaged dike can be tentatively
rehabilitated by using soils as its construction material in
comparatively short period after the earthquake, and that flooding does
not occur at the same time with earthquake.

The most important thing from the geotechnical point of view is the
estimation of the seismically induced deformation of dikes. For this
purpose, conventional stability analysis by circular arc method has been
used for a long time. In this empirical method, both safety factor
against inertia force without reducing soil strength, and safety factor
against reduced soil strength due to the liquefaction induced pore water
pressure in the liquefiable layer where the inertia force is not taken
into consideration are separately examined. And the obtained safety
factor, smaller than the other, is converted to crest settlement by
using empirical relation between the crest settlement and the safety
factor.

However it is being disclosed that the deformation mode of the
liquefied layer is different from what is assumed in the conventional
stability analysis. Therefore revision is being made for estimating the
seismically induced crest settlement. Adequacy of four numerical
analyses was examined. Those numerical analyses are: a computer code
named ALID for static analysis using softened soil concept: the Towhata
method, a static approach using the viscous liquid concept: a computer
code named LIQCA using coupled effective analysis, and a computer code
named FLIP using an uncoupled analysis (Japan Institute of Construction
Engineering 2002).

Computed deformations were compared with the dike settlements
observed during the Hokkaido-nansei-oki earthquake and the Kobe
earthquake. Figure 10 shows the result of this comparison. As seen in
this figure, the estimated settlements by analytical approach agree
fairly well, but the empirical method predicts always larger settlements
than the observed ones. This too conservative estimation by the
empirical method arises from the used relationship between the safety
factor and the settlement. It is also noticed that the static approach
by Towhata method can estimate the dike settlement well as the dynamic
approaches (LIQCA and FLIP) do.

[FIGURE 10 OMITTED]

CONCLUSION

The damage to river dikes due to past earthquakes in Japan and
their rehabilitation works were summarized. It was shown that the soil
liquefaction was the main cause of seismic failure of dikes.

Although the seismic stability of existing dike is already
diagnosed, but strengthening is delayed due to limitation of budget. In
order to reduce the necessary cost by raising the precision of
diagnosis, an adequate method to evaluate the seismic deformation of
dike is desired to be established as soon as possible so that the
effective strengthening is conducted for the necessary section to
mitigate the extensive disaster.